1 Introduction

Gamma-ray bursts in the GeV energy range can in principle be observed by ground based experiments as air shower arrays. Secondary ${\rm e}^{\pm}$ generated in the atmosphere by gamma-rays with energy as low as $E \geq 1$ GeV can be detected at the ground level, provided the detector is located at very high mountain altitude. Although the number of secondaries is too small to reconstruct the primary gamma-ray direction, a GRBs can be seen as a short time increase in the single particle counting rate.

Figure 1 shows the number of secondary charged particles reaching the ground as a function of the altitude above the sea level, generated by a gamma-ray with a zenith angle $\theta=30^{\circ}$.The curves are given for 3 different gamma-ray energies.

The background consists of secondary particles (${\rm e}^{\pm}$ and $\mu^{\pm}$)generated by charged cosmic rays. Is it worth to note that the single particle cosmic ray background increases much more slowly with the altitude than the gamma-ray signal does. As an example, going from 2000 m to 5000 m the background rate increases by factor $f_{\rm b} \sim 2-3$ (depending on the latidude), while the "signal" of a 100 GeV gamma-ray increases by a factor $f_{\rm s} \sim 30$.As a consequence the detection sensitivity increases by a factor $f_{\rm s}/\sqrt{f_{\rm b}} \sim 20$.